AltSci Blog

April 8-10, 2015
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keystream_dupe-0.6.tgz [sig]
keystream_dupe-0.5.tgz [sig]
keystream_dupe-0.4.tgz [sig]
keystream_dupe-0.3.tgz [sig]
keystream_dupe-0.2.tgz [sig]
keystream_dupe-0.1.tgz [sig]

In this very basic cryptography exercise, I have written a simple test of the quality of a cipher. For RC4 and stream ciphers, we can encrypt \x00\x00\x00\x00 to get the first four bytes of the keystream. I do this for the first 1048576 keys (assuming big endian and 64-bit keys) with RC4. Then I find out how many random keys I have to try before I find the same first four keystream bytes. I do this 1024 times. The data shows that this is around 4 million keys.

For block ciphers like AES, we have to do it slightly differently, but the concept is the exact same. I encrypt "GET / HTTP/1.1\r\n" which happens to be 16 bytes, the exactly correct size to fit in a single block of AES plaintext. I store the first four bytes of the ciphertext for the first 1048576 keys (same assumption as above but 128-bit keys). Then I do the same with random keys and I compare the first four bytes of the ciphertext against the first four bytes of the 1048576 partial ciphertexts. I find out how many random keys I have to try before I find the same first four ciphertext bytes. I do this 1024 times. The data shows that this is around 3 million keys. As you can clearly see, this is far smaller than RC4 (which is known to be vulnerable to many attacks).

To test whether the problem is in AES or RC4, I used my system's random number generator (Linux /dev/urandom) to generate random bytes of keystream and tested how many attempts it would take to collide 1024 times. It took on the order of 4 million. This proves that the issue is either in AES or in RC4 and my system's random number generator. Since my system's random number generator is as good a source of entropy as I have, I must conclude that there is no issue with RC4 and that there is an issue with AES.